One technique for reducing unwanted emissions, such as NOx, from internal combustion engines involves introducing chemically inert gases into the fresh airflow used in the subsequent combustion process. The oxygen concentration in the combustion mixture is reduced, the fuel burns slower, peak combustion temperatures are reduced and, consequently, the production of NOx gases is reduced. One way of introducing chemically inert gases is through the use of a so-called Exhaust Gas Recirculation (EGR) system.
EGR operation may be adjusted to engine operation conditions. In this case, an EGR system may include a valve, commonly referred to as an EGR valve, for the controlled introduction of exhaust gas into the intake manifold. The EGR valve operation modifies the turbocharger speed and swallowing capacity, that is the mass flow rate, of the turbocharger indirectly by changing the exhaust flow through the turbine of the turbocharger.
In addition to an EGR valve, air-handling systems for turbocharged internal combustion engines may include one or more supplemental or alternate air handling control mechanisms for directly modifying the swallowing capacity and speed of the turbocharger. For example, the air handling system may include a wastegate disposed between an inlet and outlet of the turbocharger turbine to selectively route exhaust gas around the turbine and thereby control the swallowing capacity and the speed of the turbocharger. Alternatively or additionally, the system may comprise an exhaust throttle disposed in line with the exhaust conduit either upstream or downstream of the turbocharger turbine to control swallowing capacity and the speed of the turbocharger.
Alternatively, or in addition, the turbocharger may comprise a variable geometry turbine, which is used to control the swallowing capacity of the turbocharger by controlling the geometry of the turbine. This type of turbochargers may be useful, e.g., in lean burn gas engines, where combustion is sensitive to gas quality and air temperature variations.
US 2006/0005540 A1 discloses a system for limiting the rotational speed of a turbocharger to avoid overspeed conditions and damage to the turbocharger. US 2006/0005540 A1 also discloses systems, which do not have a turbocharger rotational speed sensor.
In systems in which a turbocharger rotational speed sensor is not present, the rotational speed of the turbocharger is estimated from other types of signals. A suitably programmed microprocessor may be used to estimate the rotational speed of the turbocharger. The microprocessor may be considered as a virtual turbocharger speed sensor.
Further systems and methods that enable the rotational speed of the turbocharger to be more accurately and reliably estimated are provided. In addition, other objects, desirable features and characteristics will become apparent from the subsequent summary and detailed description, and the appended claims, taken in conjunction with the accompanying drawings and this background.